Blood-nerve barrier permeabilities to ions and nonelectrolytes is very low, indicating limited exchange between plasma nerve extracellular compartments In the rat, integrity of the blood-nerve barrier to small nonelectrolytes is maintained with age. The blood-nerve barrier, like the blood-brain barrier, has regulated carriers for the transfer of manganese, neutral amin acids, and basic amino acids. In addition, cationized albumin is taken up into nerve at a greater rate than native albumin, possibly by receptor-mediated transcytosis. Similar properties have been observed at the blood-brain barrier. A modified polymerase chain reaction titration method was developed to quantify glucose transporter mRNA expression at the cerebral microvessels. Lipid composition isolated cerebral capillaries differs with age. There are more unsaturated than saturated fatty acids in capillaries of older rats, a reduced amount of ethanolamine plasmalogen, an reduced phospholipase A2 activity.